The September 11, 2001, terrorist attack on the World Travel Center was an extraordinary environmental disaster resulting in an unprecedented combination of physical and emotional trauma to the rescue and recovery workers. As a result, over a decade later, lower respiratory symptoms (LRS) and post-traumatic stress disorder (PTSD) continue to be the signature sequelae of the World Trade Center (WTC) disaster, with as many as 60% of responders experiencing clinically significant symptoms a decade later. Our NIOSH-funded program of research is focused on the understanding the magnitude and impact of this comorbidity in WTC responders and its underlying mechanisms in order to identify more effective diagnostic biomarkers and therapeutic interventions. Our current proposal is informed by results of our ongoing epidemiologic study (NIOSH award 200- 2011-39410), where we found that PTSD is associated with LRS in WTC-Health Program (HP) responders cross-sectionally (Luft et al. 2012) and is associated with a two-fold increase in new-onset LRS 2.5 years later (Kotov et al., submitted. These findings raise important questions about mechanisms underlying the PTSD/LRS relationship. In an effort to uncover the mechanisms, we began by confirming epigenetic variations reported by Uddin et al. (2010) (NIOSH award U01OH010416). In our initial sample of 237 responders (final target N=500), we found four immune genes (DAB2IP, TAOK3, TLR9, TRAF3) to be less methylated in responders with PTSD. These genes regulate activity of NFB, a transcription factor that is a central regulator of pro-inflammatory gene expression and strongly linked to a variety of inflammatory diseases and cancers. They are highly expressed in peripheral blood leukocytes (immune cells) and integral to inflammation and immune responses. De-methylation in these genes was also associated with LRS. We are now well-positioned to leverage our prior NIOSH funded research to accomplish the following specific aims within the proposed two-year timeframe: 1) Identify differential DNA methylation patterns within the lymphocytic and monocyte immune cell population in responders with and without PTSD. We will determine whether specific WTC exposures, PTSD symptoms, comorbid psychiatric conditions, and specific respiratory disorders are associated with these epigenetic differences. 2) Relate methylation patterns associated with PTSD to cell-specific gene expression and cytokine levels in plasma to determine downstream effects of epigenetic differences. 3) Assess health outcomes of participants 2 years after the biological samples were obtained to determine whether methylation, gene expression, and levels of inflammatory cytokines contribute to onset and persistence of LRS. We also will test longitudinally whether these biological mechanisms mediate the PTSD- LRS relationship. The proposed study will be the first to examine mechanisms contributing to comorbidity between PTSD and LRS, a highly prevalent problem that persists despite conventional treatments. Elucidation of the etiologic pathway linking the two conditions will help to identify biomarkers which may be useful in diagnosis and for more specific, tailored, and effective treatments that target particular cells, cytokines, or genes that are altered in PTSD. The present study will also advance science by shedding light on effects of toxic disasters on human biology and hence the results will be relevant to other exposed populations, many of whom suffer from similar burdens.